The present invention concerns Lorentz force microscopy and in particular concerns imaging magnetic structures in a sample with high resolution by measuring Lorentz force-induced deflection of the tip in a scanning tunneling microscope (STM).
Several approaches exist for imaging magnetic field distributions on a microscopic scale. For moderate resolutions optical techniques based upon the Kerr effect are adequate and provide spatial resolution of approximately 0.5 micron which is limited by the optical wavelength. Another approach is to use the Bitter pattern technique which requires the spreading of a magnetic particle suspension over the surface to be imaged and subsequently obtaining an image using light. In order to achieve higher spatial resolution, it is necessary to resort to the use of electron beam imaging techniques such as spin polarized imaging and Lorentz microscopy.
Presently, leading techniques for high-resolution imaging of magnetic structures include the use of magnetic force microscopy (MFM) as is described in the article by Y. Martin and H.K. Wickramasinghe entitled "Magnetic Imaging by "Force Microscopy" with 1000 .ANG. Resolution", Appl. Phys. Lett Vol. 50, No. 20, May 18, 1987, pp. 1455-1457, in which the lateral resolution is limited by the tip size, typically 1000 angstroms. Another technique is the use of scanning electron microscopy with polarization analysis (SEMPA) as is described in the article by R.T. Celotta and D.T. Pierce entitled "Polarized Electron Probes of Magnetic Surfaces", Science, Vol. 234, Oct. 17, 1986, pp. 333-340, which in principle is limited in resolution to the far-field electron beam spot size. The SEMPA technique also suffers from the difficulties associated with surface preparation and in the reliability of the polarization-sensitive detector. To date, the SEMPA technique has been demonstrated with a resolution of 1000 angstroms, but the potential exists of achieving 100 angstroms resolution.
The present invention provides for accurate measurement of the force between a tip and a sample as a function of the spacing between the tip and sample surface. A tip is vibrated in close proximity to &he surface and an optical heterodyned interferometer is employed to accurately measure the vibration of the tip. The technique provides a sensitive and flexible arrangement for measuring the force. As a result, it is possible to image the magnetic field by noncontact profiling on a scale of a few angstroms.
Specifically, the measuring technique of the present invention is theoretically capable of attaining resolution which is limited only by the near-field beam size, i.e. better than 5 angstroms lateral resolution, as demonstrated using high-resolution images of a scanning tunneling microscope.
In accordance with the teachings of the present invention, a scanning tunneling microscope is operated using a long, thin tip. In such a configuration the tip is stiff in a direction normal to the plane of the sample surface but is flexible in a direction parallel to the plane of the sample surface. In the presence of a magnetic field in a plane parallel to the sample surface, and with a tunneling current passing between the tip and the sample, there will be a static deflection of the tip. When applying a first alternating current bias voltage at a first frequency between the tip and sample, the oscillatory current causes the tip to undergo vibratory motion at the first frequency in a direction parallel to the sample surface. The oscillatory motion is detected by means of an optical heterodyned interferometer by measuring the laser phase variations.
The motion of the tip may be detected in two orthogonal planes, which as will be described hereinafter determine the magnitude and direction of the component of the magnetic field parallel to the sample surface. The detection is accomplished, for example, either by using two, independent interferometers or by using a simple interferometer and measuring both the amplitude and phase of the detected optical signal. By scanning the tip across the sample surface, an image of the vector magnetic field throughout the sample is thus obtainable.
Scanning tunneling microscopes are well known and are described, for example, in U.S. Pat. No. 4,343,993 entitled "Scanning Tunneling Microscope", issued to G. Binnig et al and assigned to the same assignee as the present invention, which patent is incorporated herein by reference.